1. Field of the Invention
The present invention relates to a pneumatic tire to be attached to a motorcycle. In more detail, the present invention relates to an improvement in a tread of a tire.
2. Description of the Related Art
In cornering of a motorcycle, a centrifugal force acts on the motorcycle. For the cornering, it is necessary to apply a cornering force to be balanced with the centrifugal force. In the cornering, a rider inclines the motorcycle inward. By a camber thrust generated through the inclination, the cornering is achieved. In order to easily carry out the cornering, a tire for the motorcycle has a tread having a small radius of curvature. In straight running, a center region of the tread comes in contact with a road surface. On the other hand, in the cornering, a region provided on an outside of the center region comes in contact with the road surface.
In a race, the rider often inclines the motorcycle to the utmost limit, thereby carrying out sharp cornering. This state is referred to as a “full bank”. In the sharp cornering, a shoulder region of the tread comes in contact with the road surface.
In the cornering, a grip force is required for the tire. In other words, a high coefficient of friction between the tire and a road surface is required. By employing a soft tread, it is possible to obtain a high coefficient of friction. However, a tire having the soft tread has a poor handling stability in the straight running. The grip force and the handling stability cannot be easily consistent with each other.
Japanese Laid-Open Patent Publication No. 8-169208 has disclosed a tire for a motorcycle in which a tread includes a center region and shoulder regions positioned on both sides of the center region. The center region is hard and the shoulder regions are soft. The hard center region contributes to a handling stability in straight running and the soft shoulder regions contribute to a gripping property in cornering.
In a race, the straight running and the cornering are repeated. A transition from the straight running to the cornering and a transition from the cornering to the straight running are generated very often. In the transition from the straight running to the cornering, a contact portion is moved from the center region to the shoulder region. On the other hand, in the transition from the cornering to the straight running, the contact portion is moved from the shoulder region to the center region. When the vicinity of a boundary between both of the regions comes in contact with the road surface during the transition (that is, a transition period), a characteristic to be transmitted from the road surface to the motorcycle is changed rapidly. The change causes a behavior of the motorcycle to be unstable.
It is an object of the present invention to provide a tire for a motorcycle which is excellent in a gripping property, a handling stability and a transition characteristic.